The present invention relates generally to signal quality measuring apparatus, and, more particularly, to apparatus, and an associated method, for determining signal transmission quality levels of an information signal comprised of sequences of encoded data transmitted by a transmitter to a receiver.
A communication system is operative to transmit information between two or more locations, and includes, at a minimum, a transmitter and a receiver interconnected by a communication channel. A radio communication system is a communication system in which the transmission channel comprises a radio-frequency channel wherein the radio-frequency channel is defined by a range of frequencies of the communication spectrum.
The transmitter, which forms a portion of a radio communication system, includes circuitry for converting the information into a form suitable for transmission thereof upon a radio-frequency channel. Such circuitry includes modulation circuitry which performs a process referred to as modulation. In such a process, the information which is to be transmitted is impressed upon a radio-frequency electromagnetic wave.
The radio-frequency electromagnetic wave upon which the information is impressed is of a frequency within a range of frequencies defining in the radio-frequency channel upon which the information is to be transmitted. The radio-frequency, electromagnetic wave is commonly referred to as a "carrier signal," and the radio-frequency, electromagnetic wave, once modulated by the information signal, is referred to as a modulated signal. Such modulated signal is also sometimes referred to as an information signal, and the terms information signal and modulated signal will be used interchangeably hereinbelow to indicate the information once modulated upon the carrier.
Various modulation schemes are known for impressing the information signal upon the carrier signal to form thereby the information signal. For instance, amplitude modulation, frequency modulation, phase modulation, and combinations thereof are all modulation schemes by which information may be impressed upon a carrier wave to form an information signal.
Radio communication systems are advantageous in that no physical interconnection is required between the transmitter and the receiver; once the information is modulated to form an information signal, the information signal may be transmitted over large distances.
A two-way, radio communication system is a radio communication system, similar to the radio communication system described above, but which further permits both transmission of information to a location and transmission of information from that location. Each location of such a two-way, radio communication system contains both a transmitter and a receiver. The transmitter and receiver positioned at a single location typically comprise a unit referred to as a radio transceiver, or, more simply, a transceiver.
In some two-way, radio communication systems, the transceivers constructed to be operative therein are operative to transmit an information signal upon a first radio frequency channel and to receive an information signal transmitted upon a second, frequency channel. Because signals transmitted to and by such transceivers are transmitted upon different radio frequency channels, simultaneous two-way communication between two or more transceivers is permitted. Signals are continuously transmitted upon each of the two radio frequency channels to effectuate the two-way communication. Such two-way communication is oftentimes referred to generally by the term duplex operation of the transceiver.
Certain frequency bands of the electromagnetic frequency spectrum have been allocated for such two-way communication. For instance, a frequency band extending between 800 MHz and 900 MHz has been allocated in the United States for cellular communication systems. Other frequency bands have been similarly allocated in other countries. A plurality of radio frequency channels have been defined in such frequency band permitting operation of numerous cellular telephones (which constitute a type of transceiver construction) thereon.
In general, a cellular communication system is constructed by positioning a plurality of fixed-position transceivers, referred to as base stations, at spaced-apart locations throughout a geographical area. Such fixed-position base stations are physically connected to conventional telephonic networks. Other transceivers, typically referred to as radiotelephones, such as mobile, transportable, or portable radiotelephones, positioned within the geographic area defined by the positioning of the plurality of base stations, are operative to transmit and to receive modulated signals transmitted to and from a base station. An operator of a radiotelephone is thereby able to communicate with a fixed location of the conventional telephonic networks to which the base stations are physically connected.
The base stations comprising the cellular communication network transmit, at least periodically, data signals which are detectable by a radiotelephone. When a radiotelephone is first powered (i.e., turned on), the radiotelephone detects such data signals generated by a base station. Responsive to such data signals, transmitter and receiver circuitry of the radiotelephone is tuned to particular ones of the transmission channels allocated for the cellular communication system. Transmission and reception of voice, or other, communications then commences.
Radiotelephones operative in conventional cellular communication systems typically include apparatus for determining the signal strength of the signal received by the radiotelephone. An indication of the signal strength of signals transmitted to the radiotelephone by the base station provides an indication of the quality of the signal transmission therebetween.
In a conventional, cellular communication system, the signal strength of a received signal is the only quantitative indication of the transmission quality of the received signal.
A qualitative indication of the interference introduced upon a signal during transmission thereof to a receiver, which is also related to the quality of the signal transmission, is provided to a user of the radiotelephone as the interference distorts the signal, and such distortion affects the audible quality of the resultant signal heard by the user.
Conventionally, transceivers operative in cellular, communication systems generate modulated signals by a frequency modulation technique.
Increased popularity of use of such cellular communication systems has resulted in increased demand for access to the limited frequency bands, and, hence, limited number of radio frequency channels allocated for such systems. Schemes have been developed to utilize more efficiently the frequency bands allocated for such use.
Several of such schemes involve the modulating of the information into encoded form. In such a process, the information becomes compacted, and the information signal containing such information may be transmitted more efficiently (i.e., the same amount of information may be transmitted in a lesser amount of time). Additionally, an information signal formed by such a process need not be transmitted continuously; rather, the information signal may be transmitted in discrete bursts. One such scheme by which an information may be encoded is a .pi./4 shifted quaternary phase shift keying (.pi./4-QPSK) modulation scheme.
Various cellular communication systems have been proposed and are in various stages of implementation in which the information is encoded to permit transmission (and also reception) of an information signal formed by such a modulation scheme. For instance, in the United States, such a system has been proposed, referred to as the United States Digital Cellular (USDC) System. Also, in several other countries, and as will be described with respect to the description of the preferred embodiments hereinbelow, a somewhat similar system has also been proposed, referred to as the Group Special Mobile (GSM) System. Radiotelephones operative in such systems are similarly being developed.
In any communication system in which discretely-encoded information signals are transmitted, such as the above-noted USDC and GSM systems, the receiver which receives such signals must decode the information signal. During such decoding process, performed by decoding circuitry, interference introduced upon the signal during transmission to the radiotelephone is removed. As a result, an operator of the radiotelephone detects no audible indication of the amount of interference introduced upon the signal transmitted to the radiotelephone.
However, when the level of interference introduced upon the signal during the transmission thereof is so significant as to interfere with operation of the decoding circuitry of the radiotelephone, the operator of the radiotelephone is supplied with no signal during times in which the level of interference is so great as to prevent suitable operation of such decoding circuitry.
In contrast to a radiotelephone operative in a conventional, cellular communication system, an operator of a radiotelephone operative to transmit and to receive encoded information signals receives little or no warning of excessive levels of interference introduced upon an information signal transmitted to the radiotelephone until the radiotelephone is completely unable to decode the signal received by the radiotelephone.
While an operator of a radiotelephone operative in a conventional, cellular communication system can rely merely upon an indication of the received signal strength and a qualitative evaluation of the quality of the audible signals generated by the radiotelephone to make a judgement as to the suitability of communication by way of the radiotelephone at any particular location, an operator of a radiotelephone operative to transmit and to receive discretely-encoded information signals cannot make a similar judgement.
What is needed, therefore, is apparatus for determining a signal transmission quality level of a signal comprised of sequences of discretely encoded data transmitted to the receiver wherein the signal transmission quality level is indicative of not only the received signal strengths, but also the level of interference introduced upon the signal.